Why won't it go into overdrive?? Help quick. Experts?

Need to go on a long trip soon and want to fix this.
Lately my 2000 w/5.3 Seldom shifts into overdrive.
New filter and synthetic lube with a bottle of Lucas last Feb. Check engine light intermittently on and off.
My scanner indicates miss in #4.
Local wiz kid thought weak solenoid in tranny but used his scanner and found he could force it into OD with the scanner. 3&4 cylinders indicate frequent misses.
Fuel press. reg. new. Injectors check out, new plugs wires. Coils good.
He thought he could hear a noise that may indicate intake manifold leak (common?). Fuel sending unit goofy too.

Advice Please?

Do you think the miss is due to intake leak?
Do you think the miss causes the computer to keep the tranny out of OD? How much work to do the intake man. What's involved in replacing solenoid in tranny? Remove pan/valve body? Or more involved?

Thanks for looking.

Fix the miss fire first. Plugs, wires or whatever. Then see if you still have a trany problem.

Spray some carb cleaner around the intake and see if rpm changes. To hear an intake leak it would have to be a major one. Could be a simple broken vacuum line, they can cause weird symptoms. Like tailight said, get the misfire fixed first.

Yea, from what I've been told, any slight miss can keep these tranny's from going into overdrive, so it could be that.

Good luck, a intermittent miss can be a PITA.